Prime Minister Benjamin Netanyahu announced the overnight seizure of Beaufort Castle – a nearly 900-year-old stronghold – and instructed the military to deepen its advance.
The site carries both strategic and symbolic weight.
The castle gives Israeli forces a commanding vantage point over much of southern Lebanon and northern Israel.
Also, it is the first time Israel has held the site since May 2000, when it withdrew after an 18-year occupation.
Defence Minister Israel Katz said troops would retain it as part of Israel's security zone.
The advance came after a barrage of Hezbollah fire into northern Israel on Saturday, which prompted school closures.
Israeli troops, already operating up to the Litani River, are now pushing towards the Zaharani River, around 10 kilometres further north.
More than 40 strikes were carried out across southern Lebanon on Sunday.
One Israeli soldier was killed.
Lebanon's Prime Minister Nawaf Salam condemned the escalation on Saturday, calling it "dangerous and unprecedented".
He said Israel was not merely attacking specific targets but pursuing the wholesale destruction of towns, villages and livelihoods – amounting, he argued, to collective punishment of a civilian population in violation of international law.
He added that US-brokered negotiations with Israel had so far produced no results, but defended continuing them as "the least costly option for Lebanon".
France called for an emergency UN Security Council meeting on Monday, with Foreign Minister Jean-Noël Barrot saying nothing could justify the continuation of Israeli military operations or the deepening occupation of Lebanese territory.
The conflict has displaced more than 1.2 million Lebanese and killed more than 3,370 people, according to the country's government.
Israel says 24 soldiers and four civilians have died on its side, with tens of thousands of northern Israeli residents displaced by Hezbollah rocket and drone fire.
(ał)
Source: Reuters, IAR